Field of the Invention (Technical Field)
The present invention is in the field of fluids and the like comprising magnetic particles, such as magnetic fluids, a method of stabilizing magnetic particles, use of these fluids and functionalized particles. Such fluids have a large variety of applications, such as sealants, as a sensor, in biomedics, etc.
Description of Related Art
Various patent documents and scientific documents recite fluids comprising magnetic particles.
Magnetic Fluids are a class of smart materials that change their properties reversibly and fast (milliseconds) under presence of an external magnetic field. These fluids can show changes in apparent viscosity of several orders of magnitude when a magnetic field is applied, such as a magnetic flux density in the order of around 1 T. Below two sub-classes are identified.
A ferrofluid relates to a liquid which becomes strongly magnetized in the presence of a magnetic field. Typically ferrofluids are colloidal liquids made of nanoscale ferromagnetic, or ferrimagnetic, particles suspended in a carrier fluid (usually an organic solvent or water). Typically each tiny particle is thoroughly coated with a surfactant to inhibit clumping. A disadvantage is that larger particles can be removed from an otherwise homogeneous colloidal mixture, forming a separate clump of magnetic dust e.g. when exposed to strong magnetic fields.
A difference between ferrofluids and magnetorheological fluids (MR fluids) is the size of the particles. The particles in a ferrofluid primarily consist of nanoparticles which will not settle under normal conditions. MR fluid particles primarily consist of micrometer-scale particles which will settle over time because of the inherent density difference between the particle and its carrier fluid. These two fluids have very different applications as a result.
A problem with many prior art magnetic fluids is that these are not sufficiently stable, especially over time. Therefore they can not be stored for a longer period. Many prior art fluids are also not stable at higher temperatures, e.g. temperatures of application and production of the fluid, and at low vapor pressure. Even further, they are also not stable when in use, in particular in an external magnetic field (gradient).
Production methods of prior art fluids are typically time consuming (slow), not efficient, e.g. in terms of energy and chemicals used, and are laborious.
A further problem is that many fluids comprising magnetic particles can not withstand a high pressure difference, e.g. in sealing application. Also the fluids can not be used when a relative low external magnetic field (gradient) is present, as these fluids are not magnetic enough. Obtaining fluids can be complicated in view of stabilization procedures, a high temperature and inert atmosphere being required, and dialysis processing. Therewith application of the prior art fluids is limited.
It is noted that claims in various prior art documents with respect to e.g. densities obtained can not be obtained, sometimes already from a principle point.
An issue with prior art fluids is that they foam, especially when agitated. The problem is often not recognized as such, for instance because fluids are only used on a lab scale. However, clearly for large amounts of fluids to be produced foaming is an issue, e.g. in terms of controllability.
Some prior art methods form magnetic particles at relatively high OH− concentrations. In order to control e.g. particle size and particle composition a relatively high temperature of 80-95° C. needs to be used. Without such extra measures e.g. magnetite can not be obtained.
It is noted that coating nanoparticles per se is known in the prior art, e.g. for protection thereof. A coated particle does not inherently relate to particles that can be densified in a dispersion. Despite coating of magnetic particles per se is known, such does typically not relate to providing solutions to one or more of the present problems.
Incidentally some prior art recite methods which in view of the present invention could relate to forming a pre-dispersion having a low concentration of dispersed particles (at the most 5-7 vol. %). For instance DE 102 05 332 A1 recite a magnetic dispersion formed under argon atmosphere. Also Yu J-H et al. (J. Magnetism ad Magnetic Materials, vol. 304, September 2006, pp. e16-e18) recites an water based dispersion formed under an inert atmosphere, with an oleic acid coating which is considered unsuitable. Also Lin et al. (J. Coll. And Interface Science, Vol. 291, November 2005, pp. 411-420), recites a dispersion formed under an inert atmosphere, with a polyacrylic acid (PAA) coating having a high molecular weight and long polymeric chains which is considered unsuitable. None of these documents recites a high density dispersion. In fact the dispersions formed are considered unsuitable for high density dispersions.
The present invention therefore relates to a magnetic fluid, a method of stabilizing magnetic particles and use magnetic fluids, which overcomes one or more of the above disadvantages, without jeopardizing functionality and advantages.